The present invention relates to an electrical engine control system to realize lean fuel burn in a direct injection engine, and especially to an engine control system in which unstable burn for a period in which exhaust gas remains after an exhaust gas recirculation (hereafter referred to as EGR) operation is stooped can be avoided by modifying a target air-to-fuel ratio (hereafter referred to as A/F) during a transitional state when switching the operation state from a burn state with EGR to that without EGR.
In a conventional direct injection engine, for example, as described in Japanese Patent Application Laid-Open Hei 7 - 166916, a switching operation between the idling operation and the off-idling operation, which corresponds to a stratified charge burn, is disclosed. This conventional technique decreases an injection amount by the amount corresponding to a decrease in the pumping-loss while intake air is increased if it is determined that the operation has been switched from idling to off-idling.
Although a switching operation between the off-idling operation in a homogeneous charge burn and the off-idling operation in the stratified lean burn is disclosed, a switching operation from the stratified charge burn with EGR capable of improving a fuel consumption to the homogeneous charge burn is not considered.
Generally, in the stratified lean burn, a high burn stability can be obtained in the EGR operation, and even if a high degree of EGR is performed, a burn stability can be secured, and fuel consumption can be further improved. On the other hand, in the homogeneous lean burn, because the stability of burn is degraded in the EGR operation, stable burn is maintained by operating the engine without EGR or with the low degree of EGR.
That is, the burn instability may occur because the exhaust remains yet just after the operation is switched from the stratified lean burn with EGR to the homogeneous lean burn without EGR.